Torque limiting means for variable displacement pumps

ABSTRACT

The loading of rotor support journals, of variable displacement pumps, which is proportional to the theoretical driving torque of the pump, is converted to a fluid pressure which is proportional to the theoretical pump driving torque. The fluid pressure is applied to a plunger in opposition to an adjustable load, to operate a servo valve to regulate the pump displacement. The loading may be varied to provide variations in the pump driving torques to correspond to the torques available from a prime mover under a variety of power settings.

United States Patent Ifield Apr. 15, 1975 TORQUE LIMITING MEANS FOR3,726,609 4/1973 Cattanach 417/270 VARIABLE DISPLACEMENT PUMPS 3,733,9635/1973 Kubilos 91/506 3,750,532 8/1973 Kubilos 91/506 1 n entor: RichardJ p Ifield, 95 Copeland 3,803,987 4/1974 Knapp 91/506 Rd., Beecroft,Sidney, New South wales Austraha 2119 Primary ExaminerWilliam L. Freeh[22] Fil d; O 3 1973 Attorney, Agent, or Firm-McGlew and Tuttle [21]App]. No.: 402,973

[57] ABSTRACT [30] F i A li i P i i D The loading of rotor supportjournals, of variable dis- Oct. 23, 1972 Australia 922 72 PlacementPumps which is Proportional to the theo' retical driving torque of thepump, is converted to a 52 US. Cl 417/217; 417/222 fluid h which ispropqrhonal the themehca 51 Int. Cl. F04b 49/00 Pump dhvmg tmque- ThePressure is applied 3 [58] Field of Search 91/504-506; Phmger Oppositionan adjustable load Operate 7/217, 218 222 15 a servo valve to regulatethe pump displacement. The loading may be varied to provide variationsin the [56] References Cited pumlpbiiritving torques to corresdpond tothe torques avaia e roma rime mover un eravariet of ower UNITED STATESPATENTS settings y p 3,155,047 11/1964 Keel 91/485 3,407,738 10/1968Bosch 417/218 8 Claims, 9 Drawing Figures PATENTEUAPRI 51975 PATENTEDAPR1 5191s 3.871839 sum u q: 5

FIG. 7.

TORQUE LIMITING MEANS FOR VARIABLE DISPLACEMENT PUMPS FIELD ANDBACKGROUND OF THE INVENTION This invention relates to hydraulic pumpsand motors of the kind comprising a body, and a rotor surrounded by andmounted in an external support, the body having an inlet and outlet forliquid to be pumped and the rotor carrying a plurality of equiangularlyspaced pistons which are disposed parallel to or inclined to the rotorrotational axis and are reciprocated as the rotor rotates in order topump liquid. Such a pump will be referred to hereinafter as being of thevariable displacement piston pump type.

One form of piston pump to which the invention may be applied comprisesa rotary cylinder block assembly in which the pistons are equally spacedaround the cylinder block axis with the cylinder bores parallel to theaxis. The cylinder block is linked to the driving mechanism in such away that the driving mechanism and cylinder block rotate at the samespeed. As the shaft rotates, the distance between any one piston and theporting surface changes continually. Each piston moves away from theporting surface during one half of the revolution and toward the portingsurface during the other half, the inlet port being in line as thepiston moves away, and the outlet port being in line as the piston movescloser.

An object of the invention is to provide a variable displacement pistonpump, with torque limiting means in a convenient form, as to ensure asimplified and cheaper production of the torque means, which are easilyadjusted on site, or controlled to suit any available torque from apower supply unit.

The invention consists in torque limiting means for limiting the drivingtorque of a variable displacement piston pump comprising ahydrostatically balanced slipper bearing, supporting the pump rotor, andpressure responsive means, responsive to the hydrostatic bearingpressure, to regulate the pumps displacement.

When variable displacement piston pumps are employed for powertransmission purposes, it is usually necessary to provide a manualcontrol to regulate the maximum pump displacement and to provideoverriding devices, one to reduce the pump displacement if thetransmission pressure exceeds a predetermined value, in order to protectthe transmission components against overload, and another to reduce thepump displacement if the driving torque exceeds a predetermined value,in order to prevent overloading of the engine or electric motor whichdrives the pump.

The theoretical pump driving torque is the product of the pumpdisplacement and the pressure difference, and the torque limiting meansare based on this fact. Hitherto the torque limiting means haveincluding a plunger subject to the pump delivery pressure to obtain aservo valve to regulate the pump displacement through a servo motor, theplunger movements being opposed by a spring loading which is varied by acam responding to the movements of the servo motor, so that the springloading is inversely proportional to the pump displacement. Such torquelimiting means are complex and costly to produce and they can beadjusted correctly only on special test rigs incorporating torquedynamometers. Such torque limiters cannot be adjusted in a proportionalmanner to suit varying available torques, such as the varying torques ofpetrol engines operating over a range of throttle openings.

SUMMARY OF THE INVENTION The invention is based on an appreciation thatthe rotor support journal loading of variable displacement pumps is theproduct of the pump displacement and the operating pressure, so thejournal loading is proportional to the theoretical driving torque.According to the invention, the torque responsive journal loading isconverted to a fluid pressure, so that the fluid pressure isproportional to the theoretical pump driving torque. This pressure isapplied to a plunger in opposition to an adjustable load to operate aservo valve to regulate the pump displacement. The invention includesnovel means for varying the loading, to provide variations in the pumpdriving torques to suit the torques available from a prime mover under avariety of power settings.

The invention is described as applied to a particular form of axialpiston pump, which is controlled overcenter for forward and reversedrive, butit can equally well be applied to other forms of variabledisplacement pumps.

The invention is described with reference to a particular type ofhydro-statically balanced slipper bearing, but it can equally well beapplied to operate from any design of hydrostatically balanced bearing,where the hydro-static pressure supports the torque responsive loading.In order to illustrate the function of the invention in relation to theother controls, a suitable complete control system is described, but theinvention is not limited to the type and details of the control systemor of servo motor employed.

BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention isdescribed with reference to the accompanying drawings.

FIG. 1 is a longitudinal section through a typical axial piston pump ofa kind to which the invention may be applied.

FIG. 2 is an end view of the stationary port face for the pump shown inFIG. 1.

FIG. 3 isa diagrammatic representation of a pump control systemaccording to one embodiment of the invention.

FIG. 4 shows a preferred form of servo control valve.

FIG. 5 is a graphical representation of the performance of an enginespeed responsive torque limiter according to the invention.

FIG. 6 shows a method for achieving the control characteristics shown inFIG. 5.

FIGS. 7, 8 and 9 are sectional views taken along the lines 7-7, 88 and9-9 respectively of FIG. 4.

Referring to FIG. 1, the pump cylinder barrel 1 contains nine axialcylinders with pistons 2 having ball and socket joints 3 to flat facedslippers, bearing against a variable angle oblique surface 4 and held incontact with the surface by the slipper retaining plate 5.

The cylinder rotor is loaded against its port face 6 by a number ofsprings 7, reacting through the drive shaft 8 to the thrust bearing 9. Asuitable prime mover, such as an electric motor or an internalcombustion engine, is arranged to be coupled to drive shaft 8.

The oblique surface 4 is on a part cylindrical element 10, bearing inthe housing end cover 11 and the angle of the oblique surface isadjustable by movements of the servo motor piston 12 through a geartooth rack 13 engaging teeth formed on the part cylindrical element 10.Movement of the servo motor piston is capable of rotating thecylindrical element to an oblique angle of say 20 in either direction,thereby varying the strokes of the pistons and the direction of flowthrough the portings.

When the element 10 is in the position shown and the pump is drivenclockwise, as viewed from the end of the drive shaft 8, the deliverywill be from port A shown in FIG. 2, whichwill be referred to as theforward high pressure port. When the element 10 is tilted in theopposite direction, the delivery will be through port B, which will bereferred to as the reverse high pressure port.

The drive shaft 8 is provided with a high degree of flexibility, so thatthe cylinder barrel 1 is free to align itself against the flat port facesurface 6 and the cylinder barrel is located by four part sphericalslipper pads. The journal loading at the cylinder barrel is applied atthe slipper 14, when the oblique surface 4 is tilted in the directionshown. When the oblique surface is tilted in the opposite direction, thejournal loading would be at the slipper 15. There are no theoreticalsideways loadings at the cylinder barrel, but a slipper 16 as shown inthe partial section of FIG. 2, is placed at each side for locationpurposes only.

The slippers 14 and 15 are hydro-statically balanced against the journalloadings. They may for example be connected through the connectionpoints marked C and D to the highest porting pressure through restrictedorifices. The sealed area at the spherical faces of the slippers issufficiently great for the requirements at maximum pump displacement.The required hydrostatic balance pressure becomes progressively lessthan the porting pressure as the pump displacement is reduced. Underthese conditions the slipper operates with a small clearance at itsspherical surface, the resulting leakage providing an appropriatepressure drop at the restricted orifice.

The inside of the pump housing is maintained at the same pressure asthat in the low pressure port, so the journal loading is proportional tothe product of the porting pressure difference and the tangent of theangle of the oblique surface. The pump driving torque is alsotheoretically proportional to the product of these two variables, so thehydro-static pressure at the slipper is proportional to the theoreticaldriving torque. According to the invention, this hydro-static pressureat the loaded slipper bearing is employed as a signal for the torquelimiting device.

The two ends of the servo cylinder are shown with connection pointsmarked E and F and the various connection points identified by theletters A,B,C,D,E, and F in FIGS. 1 and 2 are similarly identified inthe control system diagram FIG. 3.

Referring to FIG. 3, a low pressure pump 17, draws from the reservoir 18and its delivery pressure is limited by the setting of the low pressurerelief valve 19. It delivers directly to the inside of the variabledisplacement piston pump housing, from which it flows to whichever isthe low pressure port through one of the non return valves 20 or 21.Under clockwise forward conditions port A is at high pressure and port Bis charged from the low pressure pump through the non return valve 20.

Ducts from the pump ports A and B have non return valves 22 and 23, thedownstream sides of which are inter-connected, supplying the selectedhighest pressure through the restrictors 24 and 25 to the hydro-.statically balanced slippers l4 and 15, respectively, through theconnection points C'and D as described with reference to FIG. 1.

The selected highest pressure from the non return valves 22 and 23 issupplied to the center gallery of a manually operated reversing valve26, which also contains a small gallery and a central drilling to oneend,

mission pump servo motor 12 via the connection points E and F,respectively. The manually operated reversing valve 26 is shown in theposition to supply the highest pressure to the servo motor connectionpointE, with the connection point F ducted through the gallery 29 to thereservoir. This causes the servo piston to move to the position ofmaximum pump displacement as shown at FIG. 1. Movement of the reversingvalve'26 outwards reverses the pressures at the connection points E andF and causes the servo piston 12 to move to the other extreme position.

The servo valve 30 contains a spool valve 31 having an enlarged piston32 at one end, sliding in a cylinder 33 and loaded by a spring 34 in theopposite direction to the position shown. The spring chamber and theremote end of the spool valve 31 are ducted to the reser voir 18. Thedelivery port of the low pressure pump 17 is ducted to the cylinder 33through the restrictor 35 to move the spool valve 31 against the loadingof the spring 34 to the position shown, where it does not influ ence theoperation of the servo motor in response to the movements of themanually operated reversin valve 27.

The galleries 36 and 37 of the servo valve 30 are ducted to thetransmission pump ports A and B respectively. If the cylinder 33 isexhausted, the spool valve '31 will move in the opposite direction tothat shown, to

isolate the galleries ducted to the manually operated reversing valve 27and to provide communication from the pump ports A and B to theconnection points F and E, respectively, of the servo motor cylinder.

Port A is the forward drive high pressure port, so

movement of the spool valve 31 causes the servo motor piston 12 toreduce the displacement of the pump motor and, for forward setting ofthe reversing valve,

port B becomes the high pressure port. Movement of the spool valve 31then causes the servo piston to increase the pump displacement, asdesired under overrun conditions.

For counter clockwise rotation of the pump, it is necessary only toreverse the connections from ports A and B to the galleries 36 and 37.

In most applications of variable displacement transmission pumps, it isnecessary to provide means to stall the pump off stroke at apredetermined maximum pres sure to prevent overloading of the pump, andalso at a predetermined maximum pump driving torque, to preventoverloading of the power plant driving the pump.

A pilot valve device 38 contains two small plungers 39 and 40 bearingagainst valves 41 and 42 which are loaded against their seatings byadjustable springs 43 and 44 to close galleries which are ducted to thecylinder 33 of the servo valve. The pilot valve discharge chambers areducted to the reservoir.

A duct supplies whichever is the higher pressure from ports A and B tothe end of the plunger 39, so that if this pressure is excessive thevalve 41 opens to vent the cylinder 33, causing the pump to stall offstroke.

The pressures at the slippers 14 and are ducted through non returnvalves 45 and 46 to the end of the plunger 40, so this plunger isresponsive to the hydrostatic pressure at whichever slipper issupporting the journal loading. The force at the plunger 40 is thereforeresponsive to the pump driving torque. If this pressure is excessive,the valve opens to vent the cylinder 33, in the servo valve 42, solimiting the pumps driving torque by variation of the pumpsdisplacement.

Pump servo systems operated by the high pressure from the pump deliveryports are prone to instability problems, particularly under high gainconditions, i.e. at small pump displacement. The invention includes astabilising device which is a function of the rate of change of pumpdisplacement. A typical embodiment of this part of the invention inshown in FIG. 3.

A piston 47 is shown with a cam roller 48, bearing against a cam 49formed on the servo motor piston. For preference the cam form is suchthat the movements of the piston 47, relative to the servo motor 12, isan inverse function of the pump displacement. The outerv end of thecylinder for piston 47 communicates through a central drilling to theinside of the pump housing and is therefore subject to the pressure fromthe low pressure pump 17 to hold the cam roller in contact with the cam.The annular cylinder chamber communicates with the spring chamber of thepilot valve 38 and is ducted to the reservoir through the restrictedorifice 51. The pressure difference across the restricted orifice 51,resulting from movements of the piston 47, acts on the sealed pistons 52and 53 to vary the loading at the valves 41 and 42.

When one of the valves 41 or 42 opens to limit the pump pressure or thepumps driving torque, the pressure difference at orifice 51 provides aforce to close the valve. A reduction in pressure at the plungers 39 or40, allowing the associated valve to close, results in a pressuredifference at orifice 51, providing a force to open the valve. Thus thestabilizing device provides a leading term, to stabilise the automaticcontrol system, from the rate of change of the pump displacement.

In many applications of variable displacement pumps, it is desired tovary the maximum pump displacement by a manual control, with maximumpressure and maximum torque overriding devices. FIG. 4 shows a valve toreplace valves 27 and 30 of FIG. 3 and which provides for manualdisplacement control.

A manually operated control lever 54 rotates a ported sleeve 55 in ahousing 56. A rotary valve element 57 inside the ported sleeve has agear tooth quadrant 58 driven by the gear 59 attached to the element 10of FIG. 1, or by rack teeth formed on the servo motor piston 12. Ineither case, the gear tooth chamber communicates with the inside of thepump housing and is subject to the pressure from the low pressure pump17. This pressure acts on an annular area of the gear driven spool 57and it is applied to a cylinder 60 on the opposite side, through arestricted orifice 61. Normally the piston in the cylinder 60 is movedto the position shown by the pressure acting on the difference in areas,but if the cylinder 60 is vented, the gear driven rotary spool moves tothe left as viewed on the drawing. This corresponds to the cylinder 33of FIG. 3, the venting valves may be similar to those described for thepilot valve 38 of FIG. 3, and the connection K is identified in FIG. 3.

Directional control selector rotary valves of this type are well known.As shown in FIGS. 4, 7, 8 and 9 they usually have galleries for eachdifferent pressure with portings through the sleeve in diametricallyopposite pairs in order to avoid side loadings on the valves.

The ports marked A and B are ducted to the transmission pump portssimilarly identified. The ports marked E and F are ducted to the ends ofthe servo motor cylinder similarly identified. The port marked G isducted to the selected highest pressure identified similarly in FIG. 3.The port marked H is connected to the reservoir, 18, which ispermanently ported to the end of the cylinder containing the valve 57and, through a central drilling, to an annular chamber having an outerdiameter equal to the outside diameter of the sleeve 55.

In its normal operating position as shown, rotation of the valve sleeveprovides communication of the servo cylinder ports E and F with theselected highest pressure port G or the reservoir port H. The resultingmovement of the servo motor piston 12 closes the ports G and H, so thatnormally the valve operates as a position responsive control. Underthese conditions the ports A and B are closed.

If either of the overriding valves 41 or 42 in FIG. 3 is opened it ventsthe cylinder 60 causing the spool valve 57 to slide axially to the left.This closes the ports H and G from communication with the valve flutesand opens the ports A and B. Port A then communicates with one servocylinder of the servo motor through port F, and port B communicates withthe other servo cylinder through port E. Thus this valve operates in thesame manner as the servo valve 30 described for FIG. 3.

The spring which loads the valve 42 for torque limitation is shown withan adjusting screw. Obviously this screw may be replaced by a push rodoperated from a camshaft coupled to the controls of a prime mover, so

that the pump driving torque would be a function of the setting of theprime mover power controls.

A preferred method of controlling the pump driving torque as a functionof the prime mover power control settings is to cause the controllingtorque to increase as a controllable function of prime mover speed untilthe torque required line intersects the available torque line from theprime mover. This is exampled by the curves in FIG. 5. The curves al, toa5 represent prime mover torque outputs for given throttle openings orgiven fuel pump discharges for diesel engines. The curves b define acontrolled torque area increasing with the square of engine speed, witha negative constant, so that there is zero torque loading for enginespeeds below l5 percent and the torque loading increases to at 40percent of maximum engine speed. The actual controlled engine conditionsare at the intersection of the 0 curves and the b curves.

The curve b, represents a minimum control setting and the curve brepresents a maximum control setting for the torque control, and anyother curve may be provided between these limits, for different primemover power requirements.

A method of achieving these desired control characteristics is shown inFIG. 6 which illustrates the torque limiting device, the low pressurepump 17, the reservoir 18 and the low pressure relief valve 19.

The lettered connection points to the torque limiter are .1, the torqueresponsive hydro-static pressure from the slipper bearing, K, the pilotpressure identified at the servo valve device 30 in FIG. 3, or thedevice illustrated in FIG. 4, and L, stabilising the pressure from thecylinder 50 in FIG. 3.

A variable orifice 62 is fitted in the discharge passage from the lowpressure pump 17. The pressure drop at this orifice is applied to thepiston 63, against the loading of the spring 64, to close the valve. Thespring 64 provides the desired negative constant. The pump 17 is drivenby the prime mover, so the pressure at J, which is proportional to thepump driving torque, will be K N -K where N is the engine speed, K isthe negative constant and K is varied with the setting of the primemover power controls, to provide controlled torque lines such as b, andb in FIG. 5. The arrangement shown in FIG. 6 can replace the equivalentarrangement 38 shown in FIG. 3.

The claims defining the invention are as follows:

1. In a variable displacement piston pump including a driven rotor,torque limiting means, for limiting the driving torque of the pump,comprising, in combination, a hydrostatically balanced slipper bearingsupporting the pump rotor, pressure responsive means operable toregulate a pump displacement, and means transmitting the hydrostaticbearing pressure to said pressure responsive means to, effect operationof said pressure responsive means.

2. Torque limiting means according to claim 1, wherein said pressureresponsive means comprise a pilot valve device including a movingelement responsive to the bearing pressure in opposition to a resilientloading, a servo motor controlling the pump discharge and a servocontrol valve operated in consequence of the operation of said pilotvalve and controlling said servo motor.

3. Torque limiting means according to claim 2, in

which the pump is driven by a prime mover, and means inder deviceconnected to the servo motor to produce a fluid pressure varying as afunction of the rate of change of the pump displacement and acting onthe pilot valve loading to enhance the stability of the control system.

5. Torque limiting means according to claim 2, wherein operation of saidpilot valve overrides the effect of the other control means acting onthe servo valve so as to limit the pump driving torque by variation ofthe pump displacement irrespective of the setting of said other controlmeans.

6. Torque limiting means according to claim 5, wherein said servo valveis a direction control selector valve, having an axially moveable spoolsuch that, at one limit of the spool axial travelythe servo valvecontrols the servo motor in response to the setting of said othercontrol means whereas, at the other limit of the spool axial travel, theservo valve causes the servo motor to reduce pump capacity; the spoolbeing moved to said other limit by a pressure change in consequence ofoperation of said pilot valve.

7. Torque limiting means according to claim 6, wherein said servo valveis a combined manual and automatic displacement and directional controlselector valve, comprising a manually operated, ported sleeve and arotary spool driven by the servo motor and subject to axial displacementin consequence of operation of said pilot valve.

8. Torque limiting means according to claim 5,

wherein said pilot valve comprises a bearing pressure responsiveplunger, bearing against a springloaded valve element, the springloadbeing mechanically ad-:

maximum torque value.

justable to regulate the =l= =l= =l TJNTTED STATES PATENT AND TRADEMARKOFFICE T11 IFIEATE QF @RRECTIQN PATENTNU. 3,877,839- Page 1 of 3 DATEDApril 15, 1975 mvtmoms) Richard Joseph Ifield It is certified that errorappears in the above-identified patent and that said Letters Patent ishereby corrected as shown below:

The title page showing the illustrative figure should be deleted toappear as per attached title page.

The sheet of drawing consisting of Fig. 3, should be deleted to appearas per attached sheet.

Column 5, line 12, "non return valves 45 and 46" should read a highpressure selector valve 45 Signed and Scaled this Fourteenth Day of May1985 tstmt Arrest:

DONALD J. QUIGG Arresting Officer Acting Commissioner 0 fPatents andTrademarks limited @taleswatent [1 1 lllielld Page 2 of 3 51 Apr. 15,1975 [54] TUEQ UE UMHTHNG MEANS FOR VAEllAElLE DTSPLACEMENT PUMPSRichard Joseph lfield, 95 Copeland Rd., Beecroft, Sidney, New SouthWales, Australia, 2119 [22] Filed: Oct. 3, 1973 [21] Appl. No.: 402,973

76] lnventor:

[30] Foreign Application Priority Data Oct, 23, 1972 Australia 922/72[52] US. Cl 417/217; 417/222 [51] lm. Cl. F041) 49/00 [58] Field ofSearch 91/504-506;

[56] References Cited UNITED STATES PATENTS 11/1964 Keel 91/485 10/1968Bosch 417/218 3,726,609 4/1973 Cattanach.-. 417/270 3,733,963 5/1973Kubilos 1/506 3,750,532 8/1973 Kubil0s..... 91/506 3,803,987 4/1974Knapp 91/506 Primary Examiner-William L. Freeh Attorney, Agent, orFirm-McGlew and Tuttle 57 ABSTRACT The loading of rotor supportjournals, of variable dis placement pumps, which is proportional to thetheoretical driving torque of the pump, is converted to a fluid pressurewhich is proportional to the theoretical pump driving torque. The fluidpressure is applied to a plunger in opposition to an adjustable load, tooperate a servo valve to regulate the pump displacement. The loading maybe varied to provide variations in the pump driving torques tocorrespond to the torques available from a prime mover under a varietyof power settings.

8 Claims, 9 Drawing Figures Page '3 of 3 Patent N 3, 877, 839 April. 151984 SHEET 3 of 5

1. In a variable displacement piston pump including a driven rotor,torque limiting means, for limiting the driving torque of the pump,comprising, in combination, a hydrostatically balanced slipper bearingsupporting the pump rotor, pressure responsive means operable toregulate a pump displacement, and means transmitting the hydrostaticbearing pressure to said pressure responsive means to effect operationof said pressure responsive means.
 2. Torque limiting means according toclaim 1, wherein said pressure responsive means comprise a pilot valvedevice including a moving element responsive to the bearing pressure inopposition to a resilient loading, a servo motor controlling the pumpdischarge and a servo control valve operated in consequence of theoperation of said pilot valve and controlling said servo motor. 3.Torque limiting means according to claim 2, in which the pump is drivenby a prime mover, and means controlling the torque as a function of thespeed of the prime mover including means applying a fixed negative loadto said moving element in combination with means applying, to saidmovable element, a load varying as a function of the prime mover speedand power control setting.
 4. Torque limiting means according to claim2, including a stabilising device comprising a piston in a cylinderdevice connected to the servo motor to produce a fluid pressure varyingas a function of the rate of change of the pump displacement and actingon the pilot valve loading to enhance the stability of the controlsystem.
 5. Torque limiting means according to claim 2, wherein operationof said pilot valve overrides the effect of the other control meansacting on the servo valve so as to limit the pump driving torque byvariation of the pump displacement irrespective of the setting of saidother control means.
 6. Torque limiting means according to claim 5,wherein said servo valve is a direction control selector valve, havingan axially moveable spool such that, at one limit of the spool axialtravel, the servo valve controls the servo motor in response to thesetting of said other control means whereas, at the other limit of thespool axial travel, the servo valve causes the servo motor to reducepump capacity; the spool being moved to said other limit by a pressurechange in consequence of operation of said pilot valve.
 7. Torquelimiting means according to claim 6, wherein said servo valve is acombined manual and automatic displacement and directional controlselector valve, comprising a manually operated, ported sleeve and arotary spool driven by the servo motor and subject to axial displacementin consequence of operation of said pilot valve.
 8. Torque limitingmeans according to claim 5, wherein said pilot valve comprises a bearingpressure responsive plunger, bearing against a springloaded valveelement, the springload being mechanically adjustable to regulate themaximum torque value.